During drilling on production of an oil or gas well, there are occasionally unwanted flows of fluid in or near the wellbore; and there are also occasionally unwanted channels open downhole where unwanted flow could take place. Examples of these problems are:
Unwanted influx of formation fluid into the wellbore
Loss of drilling fluid into fractures or vugs in the formation
Channels in cement behind casing
Holes in casing
Improperly sealing liner hangers.
A typical scenario may be presented as follows:
When an unusually high pressure formation is encountered, it may be necessary to employ drilling mud at such high weight that a formation above the high pressure zone is fractured. This fractured zone then becomes a "lost zone" into which mud flows at such a high rate that "lost circulation" occurs. The lost circulation may be so severe that it ultimately becomes impossible to maintain a column of mud above the high pressure zone sufficient to impart the necessary hydrostatic head to offset the high pressures in the high pressure zone. As this occurs, the well becomes increasingly susceptible to blowout into the lost zone or to the surface.
There are a number of things which are tried when one or another of these problems are encountered. A common solution is to force a cement slurry into the unwanted flow channel. This procedure is often successful, although sometimes multiple treatments are necessary, as long as there is no significant flow present in the unwanted channel. Cement is useless against a pre-established flow because cement has almost no flow resistance until it is set. Thus it is always necessary to stop the flow before using cement to plug the flow channel.
The hydrostatic head of various fluids is often employed to prevent or stop unwanted movement of fluids up the wellbore. In particular, most blowouts involve the uncontrolled flow of formation fluids into the wellbore and then upwards in the wellbore. This type of blowout can be controlled by injecting fluid at the proper density and rate into the wellbore at or near the point of influx. In practice, the required density and rate may be difficult to obtain.
The unwanted loss of fluids from the wellbore is often treated by injecting a slurry of fibrous, lumpy, or flaky material into the wellbore at the region of the loss. These "lost circulation materials" are intended to plug or mat over the channels through which the fluid is entering the rock.
A pasty material known as "gunk" is sometimes used as a lost circulation material and occasionally to form temporary plugs in the wellbore. Gunk is a slurry of dry powdered bentonite in diesel oil. A typical gunk recipe is 350 lb of bentonite in one bbl of diesel oil. This slurry is quite fluid when mixed and remains fluid as long as it does not contact water. Mixing gunk slurry with an approximately equal volume of water causes the clay to hydrate giving a stiff paste. If formed at the right time and at the right place, this gunk paste is an effective lost circulation and plugging material. However, since the gunk slurry will hydrate and thicken immediately upon contacting water, it must be kept dry until it has been pumped downhole to the place where a plug is desired. The mixing of the gunk slurry with water takes place downhole as the two fluids are commingled. In some cases, there is some control over the ratio of gunk slurry to water, in other cases not even this is controlled. Since gunk only achieves adequate flow resistance to form a plug within a certain range of gunk/water ratios, the performance of gunk as a plugging agent has been erratic. In particular, gunk is seldom useful for blowout control because the requirement of having the proper gunk/water ratio is difficult to satisfy.
Another technique involves placing a high density barite slurry (barium sulfate) in the annulus adjacent to the high pressure zone to provide the extra hydrostatic head needed to stop or prevent formation fluid influx. If the barite slurry remains deflocculated after placement at the bottom of the well and relatively undisturbed, the barite settles uniformly to form a hard plug. One problem with using barite to form a plug, however, is that barite's ability to form a plug varies greatly depending upon the quality of barite used. For example, it is sometimes difficult to plug a well in the presence of a significant flow movement in the wellbore. If the fluid influx is not killed immediately by the hydrostatic head of the barite slurry, the setting barite will usually not stop the unwanted flow.